Asymmetric ion pump and method



Aug. 27, 1968 B. D. JAMES ET AL 3,398,879

ASYMMETRIC ION PUMP AND METHOD Filed Oct. 7, 1966 2 Sheets-Sheet 1 W \22i/22 I9 (22 INVENTORS BRIAN DAVID JAMES THEODORE K. TOM

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ATTORNEYS Aug. 27, 1968 B. D. JAMES E ASYMMETRIC ION PUMP AND METHOD 2Sheets-Sheet 2 Filed Oct.

CONVENTIONAL DIODE P(X 10- TORR) nnunnu ANODE F/G f0 COLLECTOR ENVELOPODE 1 --D!FFERENT|AL SPUTTER ION PUMP-- 1O 2O 3O 4O 5O 6O 7O 8O HMIN.)

48 CATHODE 2 F /G. -TR|oDE- D i P P(X 10- TORR) l 30 HMIN.)

E l nw INVENTORS BRIAN DAVID JAMES THEODORE K. TOM

HMIN.)

ATTORNEYS United States Patent lice 3,398,879 ASYMMETRIC ION PUMP ANDMETHOD Brian David James, Menlo Park, and Theodore K. Tom,

Sunnyvale, Calif., assignors, by mesne assignments, to

The Perkin-Elmer Corporation, Norwalk, Conn., a corporation of New YorkFiled Oct. 7, 1966, Ser. No. 600,297 3 Claims. (Cl. 230-69) Thisapplication is a continuation-in-part of copending application Ser. No.511,516 entitled, Asymmetric Ion Pump and Method and filed Dec. 3, 1965.

This invention relates generally to a vacuum pump of the electronic typeand more particularly to so-called ion pumps, and to a method of ionpumping.

Electronic pumps employing cold cathode discharge in magnetic fields arewell known in the art. In such pumps, an electric field is providedbetween a cathode and anode placed in an uni-directional magnetic field.Electrons travelling from the cathode to the anode are deflected by themagnetic field and traverse a relatively long path. These electronscollide with the molecules of the gases to be pumped and form ions,atoms (disassociated molecules) and metastable molecules. The latter twospecies are captured chiefly at the surface of the anode by thegettering action of material which is sputtered from the cathode by ionbombardment.

Prior art ion pumps for pumping inert gases, such as argon and helium,included slotted cathode pumps, commonly called air-stable pumps; triodeion pumps; as well as diode ion pumps. Ion pumps, existing at this time,exhibit instability in the pumping of inert gases. This instabilityarises when the inert gas molecules, that have been pumped by burial inthe cathodes of the pump, are re-emitted causing a sudden rise inpressure. These pressure bursts or instabilities cause considerabledifficulty in vacuum systems and sometimes cause a vacuum system tostall and lose its vacuum altogether.

It is a general object of the present invention to provide an ion pumpcapable of effectively pumping inert and active gases stably, and amethod of ion pumping.

It is another object of the present invention to provide an ion pumpwhich has a relatively high pumping speed for inert gases.

It is another obect of the present invention to provide an ion pumpsuitable for pumping inert and active gases which is simple inconstruction and inexpensive to manufacture.

It is still a further object of the present invention to provide an ionpump which can operate at higher pressures of inert gases withoutstalling.

It is still another object of the present invention to pro vide an ionpump which can maintain a high vacuum for long periods in the presenceof small air leaks which contain inert gases.

The foregoing and other advantages of the present invention will be moreclearly apparent from the following description when taken inconjunction with the accompanying drawing.

Referring to the drawing:

FIGURE 1 is a side elevational view, partly in section, of an ion pumpincorporating the present invention;

FIGURE 2 is a top view, partly in section, of the ion pump shown inFIGURE 1;

FIGURE 3 is a top view of the anode-cathode assembly of the pump shownin FIGURES 1 and 2;

FIGURE 4 is a side elevational view of the anodecathode assembly shownin FIGURE 3;

FIGURE 5 is a schematic representation of the anodecathode assembly;

FIGURE 6 is a plan view showing a cathode plate;

3,398,879 Patented Aug. 27, 1968 FIGURE 7 is a graph showing acomparison of pumping speeds of a conventional diode ion pump and an ionpump in accordance with the present invention for helium;

FIGURE 8 is a graph showing a comparison of pumping speeds of aconventional diode ion pump and an ion pump in accordance with thepresent invention for argon;

FIGURE 9 is a graph showing a comparison of pumping speeds of a triodeion pump and an ion pump in accordance with the present invention forargon;

FIGURE 10 is a schematic representation of another embodiment of theinvention; and

FIGURE 11 is a schematic representation of a triode pump inconporatingthe present invention.

FIGURES 1 and 2. illustrate a typical ion pump incorporating the presentinvention. The ion pump includes a box-like pump chamber 11 supported bylegs 12. The chamber includes bottom wall 15, spaced side walls 13 andtop wall 14 which carries an inlet conduit 16- provided with flange 17to connect the ion pump to associated equipment.

A pair of pockets 18- are formed by channels 19 carried on opposed walls13a and 13b. These pockets communicate with the pump chamber and areeach adapted to receive an anodecathode assembly 21 to be presentlydescribed.

In the embodiment shown, bar-like permanent magnets 22, which may beferrite bars, are disposed outside of the pump chamber on each side ofthe channels. The magnets provide a uni-directional magnetic fieldacross the pockets. Three magnets 22 are disposed on each side of thepump chamber to provide a magnetic field to all of the pockets 1%. Polepieces 23 provide a shunt across the other walls of the pump chamber tocomplete the magnetic path so that the reluctance of the path isrelatively low. Thus, relatively high magnetic fields are present at thegaps which include the pockets 18.

A- typical anode-cathode assembly 21 is more clearly shown in FIGURES 3and 4. The assembly includes a cellular anode 26. The cells may have anyconvenient cross-section formed by cooperating cell walls. The anodeshown is formed by joining together a plurality of cylindrical anodemembers 27. This forms a cellular anode structure with the axis of thecylindrical members 27 disposed substantially perpendicular to the planeof the assembly. The anode 26 is supported between and spaced from apair of cathode plates or members 28. The support includes brackets 31secured to the sides of the anode assembly which engage brackets 32carried by the cathode plates. The engagement between the bracketsincludes a shield 33 and a coaxial cylindrical insulator 34. Theinsulator and shield are secured to the bracket 32 by a screw 36. Theother end of the insulator 34 receives screw 37 which engages the anodebracket 31. End brackets 40 serve to maintain the cathode plates inspaced relationship. The assembly is supported from the walls of thepump chamber by brackets 39 and electrical connection is made to theanode through tab 41.

As previously described, one such anode-cathode assembly is inserted ineach of the pockets 19 and is suitably supported from the walls of thepump chamber by the brackets 39.

Referring to FIGURE 1, electrical connection is made to the anodethrough the wall of the chamber. The connection includes a strap 42suitably connected to the tab 41 as, for example, by a screw. Connectionis also made to the anode of an adjacent assembly 21 by a strap 43interconnected between the tabs 41 of the assemblies. A conventionallead-through including stand-01f insulator and connector 44 is providedfor making electrical connection to strap 42 through the walls of thechamber.

In prior art ion pumps, the anode-cathode assembly is symmetric, thatis, the electric fields between the anode and adjacent cathodes haveequal values, and the reactive material forming the cathode members isof the same reactive material, generally titanium so that the sputteringrates from the two cathodes are substantially equal.

The anode-cathode assembly has a magnetic field H, FIGURE 5, appliedthereto in a direction substantially parallel to the walls of thecellular anode 26 and perpendicular to the cathode plates 28. A voltage+V is applied between the anode and the adjacent cathodes to set upelectric fields therebetween. Gas atoms within the chamber are ionizedby collision with electrons travelling be tween the anode and cathode.The electrons travel in spiral paths because of the magnetic field, andthe likelihood of striking a gas molecule is increased. The gasmolecules are ionized when they collide with the electrons. The positiveions are accelerated towards the adjacent cathode plate 28 by theelectric fields.

As explained above, in the usual configuration, cathodes are made oftitanium so that if the ion is of a chemically active gas such asnitrogen, it has a good chance of combining with the titanium to yield astable solid compound titanium nitride, inwhich case it is permanentlyremoved at the cathode. The ion also serves to sputter off a number oftitanium atoms from the surface. The titanium atoms travel to and aredeposited onto the walls of the cellular anode 26. The sputtered atomsserve to capture or getter the atoms and metastable molecules of theactive gases. This gettering is generally done by so-called activegettering, that is, they form compounds with these gas molecules. Theforegoing describes essentially the operation of the pump inpumpingactive gases.

On the other hand, if the ion which is produced by the collisions withthe electrons is of a chemically inert gas such as helium or argon, itdoes not combine with the titanium when it strikes the same though itmay cause sputtering. Instead, it buries itself in the titanium cathodedue to the large energy it has acquired by being accelerated by theelectric field towards the cathode. The removal of the inert gas ionfrom the gas phase, is however, not necessarily permanent. As pumpingproceeds, the surface of the titanium cathode is eroded due tosputtering, or to combination with active gas ions. This, in time, leadsto the exposure of the buried inert gas atoms. They are then reemittedinto the gas phase.

The electric fields set up between the anode and cathode dictate thatthe greatest amount of sputtering takes place under the center of thecells and the least amount of sputtering takes place directly under thewalls of the anode cells. Referring particular to FIGURE 6, there isshown schematically the appearance of a cathode plate after the pump hasbeen in use. The shaded areas indicate generally those areas of thecathode plate in which the least amount of sputtering has taken place.Thus, it is seen that inert gas atoms which bury themselves in the areaunder the center of the anode are almost certain to be subsequentlyre-emitted. Those that eventually bury themselves in the area beneaththe anode walls are less likely to be reemitted.

In accordance with the present invention, the sputtering rate of the twocathodes is different whereby there is a net transfer of metal from onecathode to another. Different sputtering rates can be achieved by makingthe cathodes with metals having different sputtering yields; that is,the number of metal atoms sputtered out for each incident ion isdifferent for the two materials. For example, one cathode plate may becopper and the other titanium. The pump would operate as follows: Theinert gas ions accelerated towards the cathodes would collide with bothcathodes, but due to the high sputtering yield of the copper compared tothe titanium, between three and six times as many copper atoms aresputtered as are titanium atoms. Some of the sputtered copper atoms aredeposited on the anode; others return to the cathode of origin; andothers are transferred to the opposite (titanium) cathode. Be-

cause of the difference in sputtering yield, there is a net transfer ofatoms from the copper cathode to the titanium cathode. These enableinert gas to be pumped by direct occlusion and also by reducing thesputter-induced reemission of previously buried inert gases. Because ofthe build-up of copper on the opposite cathode, less atoms are sputteredthan are deposited at certain areas and the net effect is to reducere-emission. Since these buried inert gas atoms are not re-emitted, thepump retains a steady positive pumping speed.

Referring more specifically to FIGURE 6, the copper will tend to collecton the titanium cathode in the shaded area 52, thereby building up 'athick deposit. There is still a sputtering of active titanium metal inthe central regions where many ions are incident. These metal atomsserve to pump the active gases.

While in the foregoing example a structure having one titanium and onecopper cathode is described, any two metals or alloys having differentsputtering yields may be used for inert gas pumping. The preferred ionpump is one in which the cathodes are made of chemically active materialhaving different sputtering yields. The chemically active material giveshigh pumping efficiency for active gases such as nitrogen, oxygen, andother components of air and yet pumps inert gas ions efficiently.Preferably, at least one cathode should be reactive.

Examples of reactive metals are vanadium, zirconium, molybdenum,tungsten, thorium, hafnium, neodymium, tantalum and their alloys, inaddition to titanium and its alloys. Generally, these reactive metalsand their alloys have a lower sputtering yield than some non-reactivemetals and their alloys. However, they have different sputter yields andthus any two will give cathodes with different sputter yield rates.Anion pump employing cathodes of titanium and tantalum has been found tobe highly satisfactory. Non-reactive low sputter yield metals arealuminum and nickel and their alloys. Metals having high sputter yieldsare zinc, cadmium, gold and silver, and their alloys, in addition tocopper and its alloys.

A pump assembly of the type shown in FIGURES 1-4 was constructed. Thepump included in alternate pockets anode-cathode assemblies ofconventional design including titanium cathodes, and anode-cathodeassemblies in accordance with the invention including one titanium andone copper cathode. The pump was operated with helium and argon leaks totest the pumping efficiency for inert gases. The pump was first operatedby energizing only the anode-cathode assemblies of conventional designfor a period of time, and then operated by energizing only theanode-cathode assemblies in ac cordance with the invention for a periodof time. Referring to FIGURE 7, the first portion of the curve shows thepressures obtained during the first period of time in the presence of ahelium leak. It is to be noted that pressure bursts periodically occur.The dotted line shows a transfer to pumping during the second period oftime in the presence of the same helium leak. It is to be observed thatthe pumping is continuous and no pressure bursts occur. FIGURE 8illustrates the results of the same test with an argon leak. FIGURE 9illustrates the results on argon when comparing a triode ion pump to thediode structure in accordance with this invention. The first portion ofthe curve shows the instability of the prior art pumps, while the lastportion shows pumping by a pump in accordance with this invention.

Asymmetrical sputter rates can also be achieved by employing the sametype of cathode surfaces but varying the spacing between the cathode andanodes whereby a higher electric field is present in one region than inthe other whereby there is a higher sputter yield in one cathode thanthe other. The asymmetrical characteristics may be obtained by applyingdifferent voltages between the anode and each of the cathode elements,again providing different electric fields and different sputteringrates. The asymmetrical characteristics can also be obtained byproviding cathodes of diiferent geometry; for example, one cathode whichis perforated to have a lower sputtering yield than the other, or onepositioned to have a dilferent sputtering yield than the other. A triodepump can also be constructed in accordance with the invention byproviding cathodes having different sputtering yields in the mannerdescribed above. The triode pump includes a grounded cellular anode 46,perforated cathodes 47 having a positive voltage applied thereto andgrounded collectors 48. The cathodes are constructed to have difierentsputtering yields.

Referring particularly to FIGURE 10, there is schematically shown ananode-cathode assembly in which the lower cathode is grounded, a voltage+V is applied between the lower cathode and the anode, and a voltage +V(where +V is less than V is applied to the upper cathode, therebyproviding a different voltage between the anode and the upper and lowercathodes to provide diiferent sputtering rates and pumping in accordancewith the invention.

Thus, it is seen that there has been provided a pump which is capable ofpumping active as well as inert gases while still maintaining arelatively simple and inexpensive construction.

We claim:

1. In an electronic vacuum pump, means providing a pump chamber, ananode, two cathodes each including reactive material associated withopposite sides of said anode, said anode and cathodes disposed in saidchamher, means for applying a voltage between said cathodes and saidanode, and means for producing a sputtering yield from one cathode whichis different than the vsputtering yield from the other cathode.

2. In an electronic vacuum pump, means providing a pump chamber, ananode, at least two cathodes, said cathodes being of different materialsselected from the group consisting of titanium, vanadium, zirconium,molybdenum, tungsten, thorium, hafnium, neodymium, tantalum and theiralloys, said anode and cathodes disposed in said chamber, and means forapplying a voltage between said cathodes and said anode.

3. In an electronic vacuum pump, means providing a pump chamber, ananode, at least two cathodes associated with said anode, one of saidcathodes including titanium and the other of said cathodes includingtantalum, said anode and cathodes being disposed in said chamber, andmeans for applying a voltage between said cathodes and said anode.

References Cited UNITED STATES PATENTS 3,091,717 5/1963 Rutherford etal. 230-69 X 3,112,863 12/1963 Brubaker et al. 230-69 3,161,802 12/1964Jepsen et al. 230-69 X 3,198,422 8/1965 Kienel 230-69 ROBERT M. WALKER,Primary Examiner.

Dedication 3,398,879.-B1ian David James, Menlo Park, and Tlwodm'e K.Tom, Sunnyvale, Calif. ASYMMETRIC ION PUMP AND METHOD. Patent dated Aug.27, 1968. Dedication filed. June 17, 1977, by the assignee, T hePerkin-E'Zmwr Uo'r'pomtio'm Hereby dedicates to the Public the entireremaining term of said patent.

[Oficial Gazette August 23, 1.977.]

1. IN AN ELECTRONIC VACUUM PUMP, MEANS PROVIDING A PUMP CHAMBER, ANANODE, TWO CATHODES EACH INCLUDING REACTIVE MATERIAL ASSOCIATED WITHOPPOSITE SIDES OF SAID ANODE, SAID ANODE AND CATHODES DISPOSED IN SAIDCHAMBER, MEANS FOR APPLYING A VOLTAGE BETWEEN SAID CATHODES AND SAIDANODE, AND MEANS FOR PRODUCING A SPUTTERING YIELD FROM ONE CATHODE WHICHIS DIFFERENT THAN THE SPUTTERING YIELD FROM THE OTHER CATHODE.